I can't help a lot with this query at the moment. I did hear or read something about this story, but it was years ago and I only vaguely remember. And of course, I don't know if the story I heard was true - that the song is about a real incident and the towns are indeed Stranorlar and Glenfin in Donegal (though I don't know if Dunluce would be ~ Portrush, Co Antrim?//anyway, you know how placenames change in songs; there's supposed to be a version of the Wild Colonial Boy in which the outlaw was born in 'Casco, Maine'. Two suggestions - ask suitable acquaintances in Belfast (I might do the same when I'm in Derry next week) and look up the Clancy Brothers Songbook to see if there are explanatory notes with the song.